Fluid-jet devices in and of themselves are well known through, e.g., U.S. Pat. Nos. 3,373,437 to Sweet et al, 3,560,988 to Krick; 3,579,721 to Kaltenbach; and 3,596,275 to Sweet. Typically, prior art fluid-jet devices provide a linear array of fluid-jet orifices formed in an orifice plate from which filaments or streams of pressurized marking fluid (e.g., ink, dye, etc.) are caused to issue from a fluid supply chamber. Individually controllable electrostatic charging electrodes are disposed downstream of the orifice plate along the so-called "drop-formation" zone. In accordance with known principles of electrostatic induction, each fluid filament is caused to assume an electrical potential opposite in polarity and related in magnitude to the electrical potential of its respective charging electrode. When a droplet of fluid is separated from the filament, this induced electrostatic potential is then trapped on and in the droplet in the form of an electrical charge. Thus, subsequent passage of the charged droplet through an electrostatic field will cause the droplet to be deflected towards a catching structure. Uncharged droplets on the other hand proceed along the normal droplet flight path and are eventually deposited upon a recording substrate.
Recently it has been proposed to utilize fluid-jet apparatus as a means to print patterns or the like on textile materials, attention being directed to commonly-owned U.S. Pat. No. 4,523,202 which is expressly incorporated herein by reference. In order to achieve fine printing of patterns on a textile substrate, it is necessary to utilize an orifice plate having at least one linear array of very small orifices sized in the range of, for example, 0.00035 to 0.020 inch diameters. As can be appreciated, such small-sized orifices will establish correspondingly small-sized droplets and thus it is necessary in order to achieve selective control over the charging and/or deflection of such droplets to place the charge and deflection electrodes as closely adjacent to the droplet streams as is structurally possible.
A problem exists, however, that during operation of the fluid-jet apparatus, structural vibrations may occur and will be evidenced by periodic vibrational displacements of the electrodes towards and away from the droplet streams. Thus, as a practical matter, the electrodes in a fluid-jet apparatus cannot be placed as closely adjacent to the fluid droplet stream as would otherwise be desired since some space must be provided between the electrode face and the droplet stream so as to compensate for the amplitude of the electrode vibration towards and away from the droplet streams. Should the electrode be placed too close to the droplet stream without providing such a compensating space, the electrode during vibration may contact the fluid droplet streams, thereby wetting the electrode surface. Such an occurrence is clearly undesirable since the charge and/or deflection functions of the electrodes would be disturbed due to short-circuiting of the electrodes by virtue of their wetted surfaces thereby deleteriously affecting charge and/or deflection control of the fluid droplets in the streams which, in turn, disadvantageously affects the resulting print quality on the substrate. It is towards a solution to the above-described problems that one embodiment of the present invention is directed.
The electrode structure of the present invention is preferably a flexible ribbon of an electrically-conductive material (e.g., stainless steel) which is tensioned between a pair of support arms so as to be laterally positionable substantially parallel to the linear array of fluid droplet streams issuing from the orifice plate. One surface of the electrode will thus be in confronting relationship to the droplet streams so as to charge droplets or deflect already charged droplets in the streams depending upon whether the electrode is used as a charge electrode or a deflection electrode, respectively.
In order to permit close mounting of the electrode in confronting relationship to the fluid droplet streams, one of the support arms is rigidly fixed (i.e., immovable) while a second support arm is pivotally mounted so as to be displaceable relative to the other, rigid support arm. A tensioning structure, (preferably including a force-adjustable compression spring) is operatively connected to the pivotal second support arm so as to cause pivotal displacement relative to the rigid support arm to maintain the flexible electrode under tension therebetween. The tensioning structure also serves to compensate for relaxation of the electrode (e.g., due to thermal expansion) and thus maintains the electrode under substantially constant tension between the pair of support arms.
Thus, the pivotal mounting of the second support arm of the present invention promotes laterally adjacent placment of the electrode in confronting relationship to the droplet streams. The pivotal second support arm also acts as a lever of sorts with the compression spring acting as its fulcrum so as to provide greater ease in tensioning of the electrode. These advantages are important for large cross-machine widths which the electrodes of the present invention must span so as to effectively operate as a component part of a fluid-jet apparatus for printing upon textile substrates, for example.
An alternative embodiment of the present invention employs an additional flexible and tensionable ground electrode or "ground shield" (preferably composed of a flexible ribbon or wire of electrically conductive material) mounted directly beneath the deflection electrode and in a substantially confronting relationship to the droplet catching structure.
Recently, it has been found that the use of this additional ground electrode solves a problem which occurs with electrodes used in fluid jet marking apparatus of the type having separate means for electrostatically charging and deflecting the fluid droplets. In particular, certain of the selectively charged droplets passing through the deflection field, i.e., droplets which normally are caught by the droplet catching structure and do not impact on the substrate, may not be deflected toward the catching structure to the extent necessary to impact on the droplet catcher face. In addition, certain of the charged droplets may be deflected into divergent or even fluctuating flight paths which could cause the droplets to miss the catcher entirely and fall to the substrate, thereby adversely effecting the desired printed pattern. On occasion, it has also been observed that certain stray droplets may be forced upwardly from the droplet deflection zone and may accumulate on the deflection and charging electrodes, thereby adversely affecting their performance.
The alternative embodiment of the electrode structure in accordance with the present invention solves the above problems relating to stray or divergent fluid droplets by providing an additional ground electrode or "ground shield" which is positioned substantially parallel to the droplet streams issuing from the orifice plate and directly below the deflection electrode.
The ground electrode is also positioned in substantially confronting alignment with the droplet catching structure. In that position, it serves to eliminate the problems resulting from stray or mis-deflected droplets by ensuring that the droplets are diverted toward the droplet catching surface and are thus caught by the catcher structure. In that regard, it is believed that the ground electrode serves to "stabilize" the deflection field along the fluid droplet path in the area below the deflection electrode but above the fluid droplet catcher. Thus, it has been found that the ground electrode should preferably be laterally aligned directly beneath the deflection field on the opposite side of the catcher face with a preferred vertical separation distance between the ground and deflection electrodes of between 1/4" and 1".
The ground electrode or "ground shield" according to the present invention is initially mounted and tensioned between the same pair of mounting arms used to laterally position and tension the deflection electrode. However, once in position, the ground electrode may be further adjusted and tensioned using a separate tension assembly operatively coupled to the common support structure for the deflection and ground electrodes. Thus, if desired, the ground shield may be selectively adjusted to a slightly different tensioned state from that of the deflection electrode.
A further embodiment of the tension assembly for the mounting arms for electrode structures in accordance with the present invention is also disclosed in which the charge, deflection and ground electrodes are tensioned using selectively adjustable tension assemblies in which a bias force is exerted laterally on one of two mounting arms to thereby maintain each electrode in a tensioned state without danger of displacement or misalignment of the electrodes during the mounting operation.
With respect to the charging and deflection electrodes, the present invention also provides for a structure which contacts the electrode at at least one position along its axial length between the pair of support arms so as to substantially increase the electrode's vibrational frequency and/or to substantially decrease the electrode's vibrational amplitude. This aspect of the present invention apparently effectively shortens the so-called "free length" of the electrode so that the electrode will exhibit the highest possible frequency of vibration and thus a corresponding decrease of the amplitude of vibration towards and away from the fluid droplet streams. By shortening the free length of the electrode, the structure of the present invention apparently effectively increases the fundamental frequency of vibration of the electrode (with a resulting decrease in the electrode's vibrational amplitude) thereby allowing closer placement of the electrode to the droplet streams than would otherwise be possible without the structure of the present invention. That is, if it is assumed that vibration of the electrode in a plane parallel to the droplet streams is negligible, then the fundamental frequency of the electrode in a plane perpendicular to the droplet streams can be expressed by: ##EQU1## where f is the fundamental frequency (cycles/sec), is the free length of the electrode (in.), F is the tension applied to the electrode (lbs.-force), and .mu. is the mass per unit length of the electrode (lbs.-sec.sup.2 /in.). Accordingly, by decreasing the free length of the electrode, the fundamental frequency is increased thereby decreasing the amplitude of the electrode's vibration towards and away from the droplet streams.
Such frequency increasing/amplitude decreasing functions are provided according to an aspect of this invention by means of at least one intermediate arm having a terminal end which contacts a portion of the charging and/or deflection electrode along its axial length between the pair of support arms when the electrode is in its tensioned state. The contact between the terminal end of the intermediate arm on the one hand and the portion of the electrode on the other hand apparently establishes a vibration node and thus shortens the "free length" of the electrode by establishing at least a pair of sublengths of the electrode between the intermediate arm and each lateral support arm.
This increase of vibrational frequency which is accomplished by the intermediate arm structures of this invention is directly contrary to "damping" structures typically provided with conventional electrode assemblies. That is, conventional electrode assemblies decrease or damp the vibrational frequency of the electrode over time as an attempt to permit closer placement of the electrode to the droplet stream. The present invention seeks just the opposite result in that an increased frequency (and thus decreased amplitude) is achieved by provision of the intermediate arm structures as briefly mentioned above.